When producing hollow articles of synthetic resins by the blow mold method, that is, by blow molding a preform blank in a subdivided blow mold, the process takes place "under certain heat conditions". In other words, the blank, upon being pressed out of the blowing head of an extruder, is transferred in segments, either continuously or intermittently, in a warm plastic state into the blow mold as a deformable blank, and is subsequently inflated from this state within the blow mold cavity into a completed hollow article. Similarly in known extrusion blowing methods, an injected preform blank is transferred by the injection-blowing method in its warm state from the injection point into the blow mold and there it is inflated into a hollow article as defined by the configuration of the blow mold cavity. In many cases, between formation of the preform blank and the blow molding station, a processing station is provided, which either cools those preform blanks which are too hot relative to the correct blow molding temperature, or reheats substantially cooled preform blanks. This method is used especially in the case of so-called "rack blowing" or "stretch blowing", wherein preform blanks are racked inside the blow mold in a longitudinal direction before being inflated, whereby a "racking" of the plastic molecules occurs, thereby obtaining better mechanical and frequently also better visual properties (for instance, higher strength and better transparency) in the completed article as compared with other methods.
Recently, however, manufacturers have returned to the earlier method of first producing perform blanks, reheating these blanks from the cold state, then feeding them into a blow mold--a method which is presently generally termed the "reheat method."
This "reheat method" has the advantage that a greater industrial output than previously attained can be achieved This is so inasmuch as the cycle time of the machine is no longer dependent on the discharge velocity of plasticized material in the extruder from the nozzle of the blowing head, it being possible for a plurality of extruders independently to produce preform blanks for one blow molding machine Another advantage resides in circumstance that with the production of hollow articles having openings, such as flasks, bottles, containers, canisters or the like, the threaded portion can be produced with greater precision by injection molding machines, the finished blank being transferred to the blow mold already threaded, thus avoiding the requirement to form the thread in the blow mold process as such. Therefore, the blow molding time can be reduced because the neck part generally has the thickest cross section in the completed hollow article and, when blow molded, it requires the most time for cooling which must be achieved before the hollow article can be safely removed from the blow mold without a likelihood of deformation.
In the reheat method, cold preform blanks removed from a supply source are reheated to the blow molding temperature, generally by infrared radiation which reheats the preform blanks in a least relatively minimal time duration as consistent with achieving same in the most nearly uniform manner. In this case, "uniform manner" means uniform heating both over the axial length of the preform blank as well as through its thickness.
It is a challenging manufacturing problem to achieve an equalized predetermined temperature throughout the entire cross section of the article because the preform blank, as it is conveyed past a heating element and rotates on its axis, is only heated on the outside. This creates a temperature gradient throughout the article's cross section which, with the subsequent stretching or racking and inflation, has a negative effect on the quality of the hollow article to be produced. The inside of the preform blank with its lower temperature during the stretching or racking and inflation reacts differently than the outer, more heated, or, so-to-speak, the "hot" side; and in being stretched out of its natural shape, during inflation, the inner portion of the preform blank resists the stretching more than the outside part. This results in the occurrence of non-flexible areas which adversely affect the strength and appearance of the hollow articles To counteract this, the outside of the preform blank (according to the state of the art) is heated by infrared radiation to a sufficiently high temperature to ensure that desirable deformation occurs in the inner part of the wall as well as the outer part. With many materials used for the production of high quality articles, such as PET, however, the outside of the preform blank should not be heated too much because this material is inclined to recrystallize at higher temperatures, causing brittleness in the finished hollow article and leading to hairline fractures or the like. For that reason, it is also known (as disclosed in U.S. Pat. No. 4,079,104 of May 14, 1978, to Dickson et al) to superimpose an air or gas cooling system on the infrared heating element in a manner that the cooling of the outer side of the wall occurs simultaneously with the heating of the blank as a whole. For instance, coolant air is blown on the outer surface of the preform blank in the space between it and the infrared radiation system, arranged parallel to the surface of the preform blank, and the preform blank, while being guided past the infrared radiating system, is rotated on its vertical axis so that while one side is heated by the infrared radiation system, the opposite side is cooled by an air stream.
This known method is not, however, without its disadvantages. It does not allow for sensitive adjustments to different conditions, depending not only upon the length and diameter of the preform blank, but also on the thickness of its walls, as well as the conditions of the surrounding manufacturing environment. For example, certain adjustments should be made depending upon whether the machine is operated during the morning shift or during the warmer afternoon shift or, on the other hand, during the cooler night shift. Especially, however, with increasing dimensions in the cross section of the preform blank, the known method entails certain risks For cross sections of about three millimeters or greater, which are not infrequently encountered in practice, to heat the inside wall adequately, the outer wall must be heated to such a high temperature that, despite the simultaneous cooling, at least some of the problems which arise with overheating cannot be avoided.
The aforesaid U.S. Pat. No. 4,079,104 of May 14, 1978, to Dickson et al and U.S. Pat. No. 4,076,071 of Feb. 28, 1978, to Rosenkranz et al are representative of the skill of the art and, to such extent, are incorporated by reference herein.